In inductive components, as for example in chokes or transformers, magnetic cores are commonly provided in order to achieve inductance values which in air-core coils (i.e. coils without any core) otherwise would be achievable only with comparatively high numbers of windings and, thus, in turn would result in very high ohmic resistances. Large inductances achievable by means of a magnetic core at relatively low numbers of windings can even be further improved by selecting a suitable core material. Thereby, the permeability, that is, the factor of inductance increase, is targeted adjusted by means of a suitable material. Applications of inductive components with a magnetic core e.g. cover power applications and converters for fuel cells and photovoltaic system.
In general, a good transformation of electrical energy into magnetic energy and vice versa is desired for inductive components with a magnetic core. For example, a very efficient transformation of electrical energy into magnetic energy and vice versa is achieved in coils of chokes and transformers by the magnetic core, since the magnetic core shows a strongly increasing magnetic flux density when applying an external magnetic field. The reason is that the magnetization of a material, when applying an external magnetic field, is increased until microscopically small magnetized domains in the magnetic core, the so-called Weiss domains, have increased to form a Weiss domain extending over the entire magnetic core. As a consequence, the magnetization of a saturated magnetic core cannot be further increased when the applied external magnetic field is increased, and the magnetic flux density in the magnetic core increases only very slowly. In other words, the magnetic permeability of the magnet core approaches the value μ=1 (i.e. the permeability of the vacuum) upon saturation of the core.
Inductive components with a magnetic core going into saturation in operation lead to disadvantages in numerous technical applications. For example, in a transformer, the efficiency of transforming electrical energy into magnetic energy and vice versa decreases when the core goes into saturation so that the efficiency of the transformer is considerably reduced. The occurrence of a saturation in the core is also undesired regarding chokes, since here, the capacity of a choke for temporarily storing energy in the form of a magnetic field drastically decreases. In particular in the course of miniaturizations, saturation limits decrease towards smaller currents and fields.
One possibility to prevent the disadvantageous effects combined with the saturation is to defer the occurrence of saturation to a later time, i.e. a time when applying higher external magnetic fields and higher currents, respectively, as it may be achieved by means of an air gap cut into a magnetic core, for example. Due to the air gap in the core, a slower increase of the magnetic flux density occurs so that the magnetic saturation only occurs at a later point in time (i.e. in case of higher external fields). The reason is that the air gap has a substantially higher magnetic resistance compared to the magnetic core and the magnetization of the material in the magnetic core is effectively impeded. Consequently, the magnetization of a magnetic core increases comparatively more slowly. Since the slow increase of the magnetization is proportional over a comparatively larger range with respect to the applied external field, for many applications this means only an insufficient increase of the field which is externally applied to the magnetic core.
Generally, inductive components are operated in a range in which it is ensured that the core does not go into saturation. Since saturation ranges in the production of magnetic cores cannot be fully taken into account due to process fluctuations, lower limits for external fields and operating currents, respectively, have to be specified in order to avoid undesired saturation during operation. However, this leads to a strong limitation of the controllability of an inductive component.
Therefore, the object is to keep losses in the controllability of inductive components as low as possible.